FIG. 7 is a schematic view showing a method for die bonding a semiconductor element employing a prior art die bonding apparatus disclosed in Japanese Published Patent Application No. 62-30394. In FIG. 7, reference numeral 1 designates an optical semiconductor element having dimensions of 300 .mu.m.times.300 .mu.m and a thickness of approximately 100 .mu.m, and including an active layer 1a. An upper surface electrode 12 comprising Au/AuGe/NiAu and having a thickness of 0.3 to 0.5 .mu.m is disposed on the upper surface of the optical semiconductor element 1. A rear surface electrode 13 comprising TiAu and having a thickness of 0.3 to 0.5 .mu.m is disposed on the rear surface of the optical semiconductor element 1. The optical semiconductor element 1 is mounted on a Si submount 3 having a thickness of 300 to 500 .mu.m with a soldering material 2 having a thickness of 0.5 to 1.0 .mu.m and-comprising Au/Sn or Au/Si. The Si submount 3 is disposed on an upper surface of a heat block 20, and the heat block 20 includes a heater 21 for heating the block to a temperature that can melt the soldering material 2.
FIG. 8(a) is a view illustrating a cross-sectional structure of a PBC laser mentioned in JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. LT-3, No. 5, October 1985 (IEEE), p. 978 as an example of the above-described optical semiconductor element 1. In FIG. 8(a), reference numeral 101 designates a p type InP substrate. A p-n-p-n type current blocking layer 102 comprising four InP layers, an n type InGaAsP layer 103, a SiO.sub.2 insulating film 104, and an n type electrode 105 are successively disposed on the p type substrate 101. A p type electrode 106 is disposed on the rear surface of the p type InP substrate 101. Reference numeral 107 designates a p type InP layer in a mesa shape.
The optical-semiconductor element 1 such as the above-described PBC laser is mounted on the Si submount 3 with soldering material 2, and the soldering material 2 is heated and melted, whereby the optical semiconductor element 1 is bonded to the Si submount 3. A plan view thereof is shown in FIG. 8(b).
The prior art die bonding method will be described.
First of all, the soldering material 2 is placed on the Si submount 3, and the optical semiconductor element 1 is placed on the soldering material 2 while controlling the position of the element 1 by observing the same with, for example, television camera or the like.
Next, the Si submount 3 on which the optical semiconductor element 1 is mounted with the soldering material 2, is placed on the heat block 20 which is previously heated by the heater 21 up to 200.degree. to 400.degree. C. at which temperature the soldering material 2 is melted, thereby raising the temperature of the soldering material 2 in several or less than ten seconds and melting the soldering material 2. The melting point of the soldering material 2 differs depending on material and weight ratio thereof of the soldering material 2. When Au/Sn is employed as the material, when the weight ratio is Au/Sn=80/20% as shown in FIG. 10(a), the material is melted at approximately 300.degree. C. In a case where Au/Si is employed as the material, when the weight percentage is Au/Si=94/6% as shown in FIG. 10(b), the material is melted at approximately 400.degree. C.
Next, the soldering material 2 is cooled, so that the soldering material 2 is solidified to bond the optical semiconductor element 1 to the Si submount 3.
Finally, as shown in FIG. 9(a), when the optical semiconductor element 1 bonded to the Si submount 3 is driven, the characteristics of the optical semiconductor element 1 are tested by a position detector 31 comprising light receiving elements 31b for receiving the laser light 30 emitted from the active layer 1a and a cable 31a for transmitting the laser light 30 received by the light receiving element 31b.
In the prior art apparatus and method for die bonding a semiconductor element as described above, the Si submount 3 on which the optical semiconductor element 1 is mounted with the soldering material 2 is placed on the heat block 20 which is heated previously up to 200.degree. to 400.degree. C., so that the optical semiconductor element 1 is also heated during die bonding, with a result that the optical semiconductor element 1 is deformed or its characteristics are deteriorated, resulting in a problem. In addition, when die bonding is performed while testing the element characteristics with the position detector 31, it is required, as shown in FIG. 9(b) that the light receiving elements 31b be located far from the heat block 20, employing an optical fiber 33 to transmit the laser light 30. Otherwise the light receiving elements 31b are destroyed by the heat so that the apparatus for test becomes unfavorably complicated.